1. Field of the Invention
The present invention relates to a printed circuit board.
2. Description of the Background Art
A flexible printed circuit board has for example an wiring layer of conductor formed on one surface of an insulating layer and a ground pattern formed on the other surface. Various circuit elements are mounted on the flexible printed circuit board. When the wiring layer transmits a high frequency signal, the input/output impedance of the circuit elements and the characteristic impedance of the wiring layer must be matched.
The characteristic impedance Z of the wiring layer is represented by the following expression:
                    Z        =                              L            C                                              (        1        )            where C represents the capacitance between the wiring layer and the ground pattern, and L represents the inductance of the wiring layer.
From Expression 1, the capacitance C between the wiring layer and the ground pattern is adjusted, so that the characteristic impedance of the wiring layer can be adjusted.
The capacitance is determined based on the area of the region in which the wiring layers and the ground pattern oppose each other, and therefore the capacitance can be adjusted by adjusting the width of the wiring layer. In this way, the characteristic impedance can be adjusted.
When the input/output impedance of the circuit elements is high, the width of the wiring layer must be reduced in order to raise the characteristic impedance. However, there is a limit to how much the width of the wiring layer can be reduced. The limit makes it difficult to raise the characteristic impedance. The capacitance may be reduced by increasing the thickness of the insulating layer. When the insulating layer has a large thickness, however, the flexibility of the flexible printed circuit board is deteriorated.
A technique of forming regular openings in the ground pattern and reducing the area of the ground pattern has been proposed (see JP 2000-77802 A and JP 5-343820 A).
When the area of the ground pattern region opposing the wiring layer is reduced, the capacitance decreases, and the characteristic impedance can be increased.
FIG. 3 is a plan view of an example of a conventional flexible printed circuit board disclosed by JP 5-343820 A, and FIG. 4 is a sectional view of the flexible printed circuit board in FIG. 3.
As shown in FIGS. 3 and 4, a plurality of wiring layers 12 are formed on one surface of an insulating layer 11, and a ground pattern 13 is formed on the other surface.
The ground pattern 13 includes a plurality of ground lines 201 and 202 orthogonal to one another. In this manner, a plurality of square openings 13a are regularly formed. The wiring layers 12 are at an angle of 45° with respect to the ground lines 201 and 202. As shown in FIG. 4, cover insulating layers 15 are formed to cover the wiring layers 12 and the ground pattern 13 on the surfaces of the base insulating layer 11 with adhesive layers 14 therebetween.
In this way, the area of the region of the ground pattern 13 opposing the wiring layers 12 can be reduced, and the characteristic impedance of the wiring layers 12 can be increased.
In the conventional flexible printed circuit board described above, when the relative position between the wiring layers 12 and the ground pattern 13 is shifted, the area of the region in which the wiring layers 12 and the ground pattern 13 oppose to one another changes. This causes the characteristic impedance of the wiring layers 12 to vary.
Therefore, there is a restriction on the position of the wiring layers 12 in design. The wiring layers 12 and the ground pattern 13 must be registered with one another with high accuracy in the manufacture.